This invention relates to a digital data transmission system of the type including a digital data transmitter connected to at least one receiver by a transmission medium, wherein the receiver is configured to receive a signal comprising a plurality of pulses, each characterized by a predetermined pulse shape.
Digital data transmission systems of the general type described above are in widespread use, for example as modems. Under certain circumstances, distortion of the transmitted signals by the metallic transmission medium may be a limiting factor. In particular, various frequency components of the transmitted signal will differ in propagation velocity, and the resulting dispersion of the transmitted signal causes a progressive alteration in pulse shape. This alteration of the transmitted pulse shape by the metallic transmission medium causes the pulse received by the receiver to differ from the predetermined pulse shape for which the receiver was designed to operate. This disparity can lead to transmission errors, and can limit either the maximum data transmission rate, the maximum range, or both.
There have been several attempts in the prior art to overcome this problem. For example, it has been suggested to provide an LC circuit on the output of the transmitter to alter the shape of the transmitted pulse before it is introduced onto the metallic transmission medium, and in this way to improve the correspondence between the pulse received by the receiver after distortion by the metallic transmission medium and the predetermined pulse shape. The publications of IEEE 802.3 10 Base T relate to one such modem using an LC circuit to predistort transmitted pulses.
Another approach is to process the received signal at the receiver in order to make it more nearly correspond to the predetermined pulse shape, a process known as receiver equalization. For many high speed data transmission systems adjacent transmitted pulses overlap in time, and this overlap may complicate the equalization of pulse shape at the receiver. It is an object of this invention to provide an improved apparatus for predistorting pulse shape at the transmitter, using digital techniques which can be implemented in a reliable, cost-effective manner, thereby increasing the maximum range and/or the maximum data rate of the system.